City of Auburn Storm Water Management Manual. Prepared for. The City of Auburn, Alabama. Prepared by. CH2M HILL Montgomery, Alabama A0.

Transcription

1 City of Auburn Storm Water Management Manual Prepared for The City of Auburn, Alabama Prepared by CH2M HILL Montgomery, Alabama April A0.PM Copy:

3 Contents Copyright...ii Acronyms...vii Definitions... viii Conversion Factors... xviii 1. Manual Purpose Related Documents and Guidance Use of Manual Submittal Requirements Environmental Considerations Plans Drainage Information Operation and Maintenance and Legal Documentation Example Problem Runoff Estimation Design Storm Information Excess Precipitation Calculation NRCS Curve Number Storm Water Runoff Determination Time of Concentration Peak Runoff Rate Runoff Hydrographs Routing of Design Storm Hydraulic Design of Storm Water Conveyance Systems Open Channels Pipes Design Criteria General Procedure Hydrologic Calculations Hydraulic Calculations Streets and Inlets Design Criteria Gutter Flow Calculations Curb Opening Inlets Grate Inlets Culverts Inlet control Outlet Control Culvert Selection MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 III

4 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Design Criteria Design Calculations Hydraulic Design of Storm Water Storage Systems Types of Storage Systems Dry Detention Basins Wet Detention Basin Extended Wet Detention Basin Constructed Wetlands General Design Considerations Public Safety Release Rate Detention Volume Depth Outlet Works Sediment Storage Outlet Protection and Rip-rap Inlet Protection and Rip-rap Operation Maintenance Considerations Flow Controls Length to Width Ratio Weir Orifices Storage Routing Manual Routing Calculations References Appendix A Example Problem Tables 2-1 Rainfall Intensity for Auburn, Alabama (in./hr) 2-2 Design Storm Volume for Auburn, Alabama 2-3 SCS Curve Numbers 2-4 Rational Method 2-5 SCS Dimensionless Unit Hydrograph Ratios and Mass Data 3-1 Recommended Manning n Values for Channels with Bare Soil and Vegetative Linings 3-2 Recommended Manning n Values for Channels with Rigid and Semi-Rigid Linings 3-3 Allowable Velocities for Different Soils 3-4 Maximum Velocities for Various Lining Types 3-5 Values of K 2 for Determining Loss of Head Due to Sudden Expansion in Pipes from the Formula H 2 = K 2 (V 1 2/2g) MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 IV

5 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL 3-6 Values of K 3 for Determining Loss of Head Due to Sudden Contraction in Pipes from the Formula H 3 = K 3 (V 2 2/2g) 3-7 Head Loss Coefficients for Manholes/Junctions 3-8 Hydraulic Capacities of Auburn Standard 2.0-foot Curb and Gutter for Residential Pavement Sections 3-9 Hydraulic Intake for Inlets 3-10 Culvert Entrance Loss Coefficients 4-1 Example Tabulation of Storage Characteristics Curve 4-2 Storage Indication Method Example 4.2 Calculations Figures 1-1A Standard Submittal Forms Design Information Form 1-1B Standard Submittal Forms Pre-development Conditions Form 1-1C Standard Submittal Forms Post-development Conditions Form 1-2 SW Drainage System and Subdivision Layout 2-1 Worksheet: Time of Concentration or Travel Time 2-2 Peak Runoff Rate Methodology Flow Chart 2-3 General Hydrograph Terminology 2-4 Definition Sketch of the SCS Dimensionless Unit Hydrograph 2-5 SCS Dimensionless Unit Hydrograph and Mass Curve 3-1 Open Channel Flow in a Closed Conduit 3-2 Pressure Flow in a Closed Conduit 3-3 Determination of Pressure vs. Open Channel Flow Conditions in Storm Sewer Systems 3-4 Efficient Manhole Shaping 3-5 Nomograph for Flow in Triangular Gutter Sections 3-6 Ratio of Flow to Total Gutter Flow 3-7 General Culvert Design Flow Chart 3-8 Culvert Design Form 3-9 Inlet Control Chart for Concrete Pipe Culverts 3-10 Outlet Control Chart for Circular Concrete 3-11 Inlet Control Chart for Circular CMP Culverts 3-12 Outlet Control Chart for Circular CMP Culverts Flowing Full 3-13 Outlet Control Chart for Structural Plate CMP Culverts Flowing Full 3-14 Inlet Control Chart for Concrete Box Culverts 3-15 Outlet Control Chart for Concrete Box Culverts Flowing Full 3-16 Inlet Control Chart for Oval Concrete Pipe Culverts Long Axis Horizontal 3-17 Outlet Control Chart for Oval Concrete Pipe Culverts Long Axis Horizontal or Vertical 3-18 Inlet Control Chart for Oval Concrete Pipe Culverts Long Axis Vertical 3-19 Inlet Control Chart for CMP Arch Culverts 3-20 Outlet Control Chart for CMP Arch Culverts Flowing Full 3-21 Outlet Control Chart for Structural Plate CMP Arch Culverts (18-inch Corner Radius) Flowing Full 3-22 Inlet Control Chart for Circular Pipe Culverts with Beveled Ring 3-23 Inlet Control Chart for Arch Concrete Pipe Culverts MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 V

6 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL 3-24 Outlet Control Chart for Arch Concrete Pipe Culverts Flowing Full 3-25 Critical Depth Chart for Rectangular Sections 3-26 Critical Depth Chart for Circular Pipe 3-27 Critical Depth Chart for Oval Concrete Pipe Long Axis Vertical 3-28 Critical Depth Chart for Oval Concrete Pipe Long Axis Horizontal 3-29 Critical Depth Chart for Standard CMP Arch 3-30 Critical Depth Chart for Structural Plate CMP Arch 3-31 Critical Depth Chart for Arch Concrete Pipe Sizes 15 ½ x 26 to 45 x Critical Depth Chart for Arch Concrete Pipe Sizes 54 x 88 to 106 x Discharge Coefficients for Roadway Overtopping 4-1 Methods for Extending Detention Times in Wet Ponds 4-2 Design of Outlet Protection From a Round Pipe Flowing Full Minimum Tailwater Condition (Tw<0.5 Diameter) 4-3 Design of Outlet Protection From a Round Pipe Flowing Full Maximum Tailwater Condition (Tw<0.5 Diameter) 4-4 Recommended Outfall Structure for Example Example Stage-Storage Curve 4-6 Example Stage-Discharge Curve 4-7 Example Storage Characteristics Curve MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 VI

7 Acronyms ADEM AHW ALDOT cfs CMP CN DOT FEMA FHWA fps ft ft/ft ft/sec ft/sec 2 ft 2 ft 3 HDPE HDS HEC HW L/W mg mm msl NGVD NRCS SCS SWMM Alabama Department of Environmental Management Allowable headwater Alabama Department of Transportation Cubic feet per second Corrugated metal pipe Curve number U.S. Department of Transportation Federal Emergency Management Agency Federal Highway Administration Feet per second Feet Feet per foot Feet per second Feet per second squared Square feet Cubic feet High-density polyethylene Hydraulic Design Services Hydraulic Engineering Circular Headwater Length and width Million gallons Millimeter mean sea level National geodetic vertical datum Natural Resource Conservation Service Soil Conservation Service (former name of NRCS) Storm water management manual MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 VII

8 Definitions Abstractions Access Spacing Allowable Headwater Depth Antecedent Soil Moisture Conditions Apron Backwater Baffle Wall Bank Storage Base Flood Elevation Berm The portion of a storm s total precipitation that does not become storm water runoff. The point in the pipeline where access is available from the surface, like a manhole or inlet. Maximum depth of flow allowed at any point along the ditch profile, measured from the invert, minus the required freeboard. Soil moisture at the onset of a rainfall event. A platform below a storm drain outlet to protect against erosion. Water backed up or retarded in its course, compared with its normal or natural condition of flow. In stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by the flooding of the stream below. The difference between the observed stage and that indicated by the stagedischarge relation is reported as backwater. A flat board or plate, deflector, guide, or similar device constructed or placed in flowing water or storm water storage systems to cause more uniform flow velocities, and to divert or guide liquids. The water absorbed into the banks of a stream, lake, or reservoir, when the stage rises above the water table in the bank formations, then returns to the channel as effluent seepage when the stage falls below the water table. Bank storage may be returned in whole or in part as seepage back to the water body when the level of the surface water returns to a lower level. The height of the base flood, usually in feet, in relation to the National Geodetic Vertical Datum of 1929, the North American Vertical Datum of 1988, or other datum, or depth of the base flood, usually in feet, above the ground surface. 1) A narrow ledge or path as at the top or bottom of a slope or stream bank. 2) A horizontal step or bench in the upstream or downstream face of an embankment dam. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 VIII

9 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL C Factor Channel Capacity Channel Storage Volume Constructed Wetland Control Elevation Crest Critical Depth The runoff coefficient C is a critical element in that it serves the function of converting the average rainfall rate of a particular recurrence interval to the peak runoff intensity of the same frequency. The magnitude will be affected by antecedent moisture condition, ground slope, ground cover, depression storage, soil moisture, shape of drainage area, overland flow velocity, intensity of rain, etc. If the area contains multiple types of surfaces, a composite coefficient is determined by estimating the fraction of each type of surface within the total area, multiplying each fraction by the appropriate coefficient for that type of surface, and then summing up the product for all types of surfaces. The maximum rate of flow that may occur in a stream without causing overbank flooding; the maximum flow that can pass through a channel without overflowing the banks. The volume of water at a given time in the channel or over the floodplain of the streams in a drainage basin or river reach. Channel storage is sometimes significant during the progress of a flood event. Wetlands constructed specifically for the purpose of treating wastewater effluent before re-entering a stream or other body of water or being allowed to percolate into the groundwater. A location in the receiving drainage system where the water surface elevation is known. The highest elevation reached by flood waters flowing in a channel, as in crest stage or flood stage. The depth of water flowing in an open channel or conduit under conditions of critical flow at which specific energy is a minimum for a given discharge. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 IX

10 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Critical Flow Crown Curve Number (CN) Deflectors Design Flood Conditions Design Storm Design Storm Flows Detention Area Detention Basin (Pond) Dry Detention Basin (Pond) Critical flow occurs when the flow velocity in a channel equals the wave velocity generated by a disturbance or obstruction. In this condition, the Froude number (Fr) = 1. When the wave velocity exceeds the flow velocity (Fr is less than 1), waves can flow upstream, water can pond behind an obstruction, and the flow is said to be subcritical or tranquil. When Fr is greater than 1, waves cannot be generated upstream and the flow is said to be supercritical, rapid, or shooting. In this condition, a standing wave is formed over obstructions in the river bed. In nature, supercritical flow is found only in rapids and waterfalls, but it is often created artificially by weirs and flumes with the aim of measuring discharge. The vertex of an arch or arched surface. Center of roadway elevated above the sides. A number between 0 and 100 that indicates the runoff-producing potential of a soil/vegetation combination when the ground is not frozen. A plate, baffle, or the like that diverts the flow of a forward-moving stream. The flood magnitude selected for use as a criterion in designing flood control works. The largest flood that a given project is designed to pass safely. In dam design and construction, the reservoir inflow-outflow hydrograph used to estimate the spillway discharge capacity requirement and corresponding maximum surcharge elevation in the reservoir. The rainfall or precipitation amount and distribution adopted over a given drainage area, used in determining the Design Flood. A storm whose magnitude, rate, and intensity do not exceed the design load for a storm drainage system or flood protection project. An area for the slowing and storage of storm water runoff. A relatively small storage lagoon for slowing storm water runoff, generally filled with water for only a short period of time after a heavy rainfall. Offers temporary storage accompanied by the controlled release of stored water. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 X

11 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Energy Dissipation Erosion Excess Precipitation Extended Wet Detention Basin Falling Limb Flood Elevation Profile Flood Routing Forebay Freeboard Grade Gravity Flow Headwater Depth Any loss of energy due to change in flow paths, generally by conversion into heat; quantitatively, the rate at which this loss occurs. The process by which rain, running water, waves, moving ice, and wind dislodge the upper layers of soil. As usually employed, the term includes weathering, solution, corrosion, and transportation. That portion of total precipitation that becomes storm water runoff during a storm event. Combines the treatment concept of the dry detention pond and the wet pond. The treatment volume is divided between the permanent pool and detention storage provided above the permanent pool. The portion of the hydrograph immediately following the peak and reflecting the decreasing production of storm flow. The height of flood waters above an elevation datum plane. The process of determining progressively downstream the timing and stage of a flood at successive points along a river. Also, the determination of the attenuating effect of storage on a flood passing through a valley, channel, or reservoir. The water behind a dam. A storage basin for regulating water for percolation into groundwater basins. The vertical distance between a design maximum water level and the top of a structure such as a channel, dike, floodwall, dam, or other control surface. The freeboard is a safety factor intended to accommodate the possible effect of unpredictable obstructions, such as ice accumulations and debris blockage, that could increase stages above the design water surface. The slope of a stream bed, road, curb, gutter, etc. The downhill flow of water through a system of pipes, generated by the force of gravity. The water level upstream of a culvert or bridge. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XI

12 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Hydraulic Gradient Hydraulic Radius Hydrograph Hydrologic Soil Group Hyetograph Imperviousness Infiltration Capacity Rate Interception Infiltration Interceptor Ditch Invert Jet Lag Time The gradient or slope of a water table or piezometric surface in the direction of the greatest slope, generally expressed in feet per mile or feet per feet. Specifically, the change in static head per unit of distance in a given direction, generally the direction of the maximum rate of decrease in head. The difference in hydraulic heads (h 1 h 2 ), divided by the distance (L) along the flowpath, or expressed in percentage terms: I = (h 1 h 2 )/L x 100. The cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also referred to as hydraulic mean depth. A graph showing stage, flow, velocity, or other hydraulic properties of water with respect to time for a particular point on a stream. The classification of soils by their reference to the intake rate of infiltration of water, which is influenced by texture, organic matter content, stability of the soil aggregates, and soil horizon development. A chart showing the distribution of rainfall over a particular period of time or a particular geographic area. The portion of a subbasin, sub-watershed, or watershed, expressed as a percentage, that is covered by surfaces such as roof tops, parking lots, sidewalks, driveways, streets, and highways. Impervious surfaces are important because they will not absorb rainfall, and therefore, cause almost all of the rainfall to appear as surface runoff. The maximum rate at which the soil, when in a given condition, can absorb falling rain or melting snow. The process whereby the downward movement of precipitation is interrupted and redistributed. A ditch that collects rainfall, allowing it to evaporate without contributing to runoff. The floor or bottom of a conduit. A forceful stream of fluid discharged from a narrow opening or a nozzle. The time from the center of mass of the rainfall excess to the runoff hydrograph peak. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XII

13 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Land Use Littoral Zone Natural Resource Conservation Service (NRCS) Nomograph NRCS Curve Number (CN) Method Open Channel Ordinate Orifice Overtopping Parapets Partially Full Subcritical Flow Peak Flow The primary or primary and secondary uses of land, such as cropland, woodland, pastureland, etc. The description of a particular land use should convey the dominant character of a geographic area, and thereby establish the types of activities that are most appropriate and compatible with primary uses. The region along the shore of a non-flowing body of water, corresponding to riparian for a flowing body of water. An agency of the U.S. Department of Agriculture, the Natural Resources Conservation Service (NRCS) was started in 1929 as an emergency act of Congress in response to the famous Dust Bowl when land practices caused extensive soil erosion and threatened the food production of the United States. Currently, the NRCS works in 3 areas: 1) soil and water conservation; 2) resource inventories; and 3) rural community development. A chart that represents an equation containing three variables by means of three scales so that a straight line cuts the three scales in values of the three variables, thus satisfying the equation. Relates soil type, soil cover, land use type, and antecedent moisture conditions to a curve number. Used to determine the depth of runoff for a given area. A system of conveyance channels in which the top flow boundary is a free surface (e.g., canal systems). The perpendicular distance of a point (x,y) of the plane from the x-axis. As used in water studies, an opening with a closed perimeter, usually sharp edged, and of regular form in a plate wall or partition through which water may flow. An orifice is used for the measurement or control of water. To rise above; exceed in height; tower over. A solid wall built along the top of a dam for ornament, for the safety of vehicles and pedestrians, or to prevent overtopping. A flow condition where the velocity is less than the critical velocity and the depth is greater than the critical depth. The maximum flow rate of the hydrograph. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XIII

14 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Peat Bog A wet overwhelmingly vegetative substratum that lacks drainage and where humic and other acids give rise to modifications of plant structure and function. Bogs depend primarily on precipitation for their water source, and are usually acidic and rich in plant residue with a conspicuous mat of living green moss. Only a restricted group of plants, mostly mychorrhizal (fungi, heaths, orchids, and saprophytes), can tolerate bog conditions. Point of Inflection Assumed to mark on the recession curve of a hydrograph when surface inflow to the channel system ceases. Ponding The natural formation of a pond in a stream by an interruption of the normal streamflow. Principle of Linearity The property whereby a mathematical system is well behaved (in the context of the given system) with regard to addition and scalar multiplication. Principle of Superposition A general principle applying to many physical systems that states that if a number of independent influences act on the system, the resultant influence is the sum of the individual influences acting separately. Principle of Time Invariance A system in which all quantities governing the system s behavior remain constant with time, so that the system s response to a given input does not depend on the time it is applied. Rainfall Excess Hyetograph A single block of rainfall excess over duration, D. Rational Method A simple procedure for calculating the direct precipitation peak runoff from a watershed, using the rainfall intensity, the area of the watershed, and the runoff coefficient appropriate for the type of watershed runoff surface. Recession Curve A hydrograph that shows the decreasing rate of runoff following a period of rain or snowmelt. Because direct runoff and base runoff recede at different rates, separate curves, called direct runoff recession curves, are generally drawn. Retention Pond A permanent pond used to slow storm water runoff and promote infiltration into the groundwater. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XIV

15 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Return Period Riser Rising Limb Scupper Sedimentation Sheet Flow Shoring Siltation Skew Angle Spread Storm Water Phase II Subbasin Subdivision In statistical analysis of hydrologic data, based on the assumption that observations are equally spaced in time with the interval between two successive observations as a unit of time, the return period is the reciprocal of 1 minus the probability of a value equal to or less than a certain value; it is the mean number of such time units necessary to obtain a value equal to or greater than a certain value one time. For example, with a 1-year interval between observations, a return period of 100 years means that, on average, an event of this magnitude or greater is not expected to occur more often than once in 100 years. A vertical pipe used for drainage. The increasing portion of the storm hydrograph. Contrast to Falling Limb. An opening for draining off water, as from a floor or the roof of a building. Strictly, the act or process of depositing sediment from suspension in water. Broadly, all the processes whereby particles of rock material are accumulated to form sedimentary deposits. Sedimentation, as commonly used, involves not only aqueous but also glacial, aeolian, and organic agents. An overland flow or downslope movement of water taking the form of a thin continuous film over relatively smooth soil or rock surfaces and not concentrated into channels. Providing temporary support with shores to a building or an excavation. The deposition of finely divided soil and rock particles upon the bottom of stream and river beds and in reservoirs. Deviating from rectangularity or a straight line. The width of water transported on the pavement measured from the face of the curb. The federal regulations requiring smaller communities to address storm water management and requiring coverage by a National Pollutant Discharge Elimination System (NPDES) permit. A portion of a sub-region or basin drained by a single stream or group of minor streams. A piece of land resulting from this; especially, a large tract subdivided into small parcels for sale. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XV

16 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Subgrade Substrate Surface Sump Surface Detention Surface Storage Tailwater Time Base Time Lag Time of Concentration Time to Peak Turning Line Turnout Unit Duration Unit Hydrograph Watershed The soil or rock leveled off to support the foundation of a structure. Any naturally occurring immersed or submersed solid surface, such as a rock or tree, upon which an organism lives. A low-lying place, such as a pit, that receives drainage. That part of the rain that remains on the ground surface during rain and either runs off or infiltrates after the rain ends; surface detention does not include Depression Storage. The part of precipitation retained temporarily at the ground surface as interception or depression storage so that it does not appear as infiltration or surface runoff either during the rainfall period or shortly thereafter. Water in a river or channel, immediately downstream from a structure. The total time from when runoff begins to the estimated peak flow rate. The time it takes a flood wave to move downstream. The time required for water to flow from the hydraulically farthest point on the watershed to the gauging station, culvert, or other point of interest. The time from the start of the hydrograph to the peak flow. A temporary line whose elevation is determined by additions and subtractions of backsights and foresights respectively. A structure that diverts water from a drainageway to a distribution system or delivery point. Turnouts are used at the head of laterals. The time over which 1 inch of surface runoff is distributed for unit hydrograph theory. A runoff hydrograph that is produced by 1 inch (25.4 millimeters [mm]) of excess rainfall distributed uniformly over a watershed and occurring at a uniform rate during a specified period of time. An area that, because of topographic slope, contributes water to a specified surface water drainage system, such as a stream or river. An area confined by a topographic divide that drains a given stream or river. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XVI

17 CITY OF AUBURN STORM WATER MANAGEMENT MANUAL Weir Wet Detention Basin (Pond) A device for determining the quantity of water flowing over it from measurements of the depth of water over the crest or sill and known dimensions of the device. Constructed basins that have a permanent pool of water throughout the year or wet season and generally are found in locations where groundwater is high and/or percolation is poor. MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XVII

18 Conversion Factors To convert Into Multiply By Acres square feet 43,560.0 Acres square miles EE-3 Cubic feet gallons Cubic feet/sec gallons/min Days seconds 86,400.0 Feet miles (statute) EE-4 Gallons cubic feet Gallons/min. cubic feet/sec EE-3 Miles (statute) feet 5,280.0 Seconds days EE-5 Square feet acres EE-5 Square miles acres 640 MGM02-R3/017.DOC LATEST REVISION: APRIL 30, 2003 XVIII

19 1. Manual Purpose

20 1. Manual Purpose The City of Auburn, Alabama, has been identified by the Alabama Department of Environmental Management (ADEM) as a Storm Water Phase II entity. One requirement of the Phase II program is to develop and implement a storm water management program. The City of Auburn has decided to develop an Auburn Storm Water Management Manual (SWMM) as a component of its storm water management program. The purpose of the SWMM is to provide storm water design information for local agencies, engineers, developers, or others whose activities affect storm water management in the City of Auburn. The manual will serve as a guide for city staff, consultants, and citizens to achieve consistency in the design and compliance of storm water projects so that both growth and environmental guidelines can be followed effectively. This manual has been prepared to document the following items: Runoff estimation Hydraulic design of storm water conveyance systems Hydraulic design of storm water storage systems This manual should assist all parties involved in development projects to achieve an understanding of the requirements, guidelines, and benefits of a detailed storm water management manual. Incorporating the guidelines contained in this manual into applications and permits will aid in obtaining approval for grading, erosion and sedimentation control, construction, and subdivision development permits from the Public Works Department. 1.1 Related Documents and Guidance The following documents are incorporated into this manual as references: Chapter 7 of the Auburn City Code, Drainage and Flood Control City of Auburn 2000 Zoning Ordinance (most recent version) Most recently adopted version of Subdivision Regulation for Auburn, Alabama Most recently adopted version of ALOA Erosion and Sediment Control Policy City of Auburn Standard Details and Specifications (most recent version) State of Alabama Highway Department Hydraulic Manual These references are to be used as guides in conjunction with this manual to fulfill the requirements and guidelines set by the City of Auburn, Lee County, and ADEM. 1.2 Use of Manual As stated, this manual is to be used to streamline the permit application and design process, with a typical permit application review time for the City of about 2 weeks. Other design MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

21 1. MANUAL PURPOSE approaches may be used and submitted with the understanding that the proposed approach may be denied and/or review times prolonged. 1.3 Submittal Requirements The information listed below represents the level of information that is usually required to evaluate an application. The level of information required for a specific project will vary depending on the nature and location of the site and activity proposed. Providing a greater level of detail will reduce the need to submit additional information at a later date. If an item does not apply to your project, proceed to the next item. Please submit all information on paper no larger than 24 inches x 36 inches Environmental Considerations a. Describe how boundaries of wetlands or other surface waters were determined. If there has ever been a jurisdictional declaratory statement, a formal wetland determination, a formal determination, a validated informal determination, or a revalidated jurisdictional determination, provide the identifying number. b. Provide a description of how water quantity, quality, hydroperiod, and habitat will be maintained in onsite wetlands and other surface waters that will be preserved or will remain undisturbed. c. Provide a narrative description of any proposed mitigation plans, including purpose, maintenance, monitoring, construction sequence and techniques, and estimated costs. d. Provide impact summary tables showing the amount of onsite wetlands, affected wetlands, and mitigation areas Plans Provide a map(s) of the project area and vicinity at a scale of no greater than 1 =400. Provide clear, detailed plans for the system including plan (overhead) views, cross sections (with the locations of the cross sections shown on the corresponding plan view), and profile (longitudinal) views of the proposed project as required by the City Engineer. If requested by the City Engineer, specifications also will be provided. The plans must be signed and sealed by an appropriate registered professional as required by law. Plans must include a scale and a north arrow. These plans should show the following: a. Project area boundary and total land area, including distances and orientation from roads or other landmark. b. Existing land use and land cover (acreage and percentages), and onsite natural communities, including wetlands and other surface waters, aquatic communities, and uplands. c. The existing topography extending at least 100 feet (ft) off the project area, including adjacent wetlands and other surface waters. All topography shall include the location and a description of known benchmarks, referenced to national geodetic vertical datum (NGVD). MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

22 1. MANUAL PURPOSE d. If the project is in the known floodplain of a stream or other water course, identify the floodplain boundary and approximate flooding elevation. Identify the 100-year flood elevation and floodplain boundary of any lake, stream, or other watercourse located on or adjacent to the site. Minimum floor elevations must be provided for lots adjacent to floodplains. e. The boundaries of wetlands and other surface waters within the project area. Distinguish those wetlands and other surface waters that have been delineated by any binding jurisdictional determination. f. Proposed land use, land cover, and natural communities (acreage and percentages), including wetlands and other surface waters, undisturbed uplands, aquatic communities, impervious surface, and detention ponds. g. Proposed buffer zones. h. Pre- and post-development drainage patterns and basin boundaries showing the direction of flows, including any offsite runoff being routed through or around the system; and connections between wetlands and other surface waters. i. Location of all detention ponds with details of size, side slopes, and designed water depths. j. Location and details of all water control structures, control elevations, any seasonal water level regulation schedules, and the location and description of benchmarks (minimum of one per structure). k. Location, dimensions, and elevations of all proposed structures, including utility lines, roads, and buildings. l. Location, size, and design capacity of the internal storm water conveyance facilities. m. Rights-of-way and easements for the system, including all onsite and offsite areas to be reserved for storm water management purposes, and rights-of-way and easements for the existing drainage system, if any. n. Receiving waters or surface water management systems into which storm water runoff from the developed site will be discharged. o. Location and details of the erosion, sediment, and turbidity control measures to be implemented during each phase of construction and all permanent control measures to be implemented in post-development conditions. p. Location, grading, design water levels, and planting details of all mitigation areas. q. Site grading details, including perimeter site grading. r. Disposal site for any excavated material, including temporary and permanent disposal sites. s. Dewatering plan details. MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

23 1. MANUAL PURPOSE t. Location and description of any offsite existing features (such as wetland and other surface waters, detention ponds, and building or other structures) that might be affected by or affect the proposed construction or development. u. For phased projects, provide a master development plan. Upon completion of the project, the owner shall submit the final as-built plan and details certified by an appropriate registered professional as detailed in the City s Subdivision Regulations Drainage Information a. The City of Auburn has standard forms to provide design pre-development, and postdevelopment calculations (copies shown in Figure 1-1) that need to be completed, signed, and sealed by an appropriate registered professional, as follows: 1. Runoff characteristics, including area, runoff curve number or runoff coefficient, and time of concentration for each drainage basin. 2. Water table elevations (normal and seasonal high), including area extent and magnitude of any proposed water table drawdown. 3. Receiving water elevations (normal, wet season, and design storm). 4. Design storms used including rainfall depth, duration, frequency, and distribution. 5. Runoff hydrograph(s) for each drainage basin, for all required design storm event(s). 6. Stage-storage computations for any area such as a reservoir, closed basin, detention area, or channel used in storage routing. 7. Stage-discharge computations for any storage areas at a selected control point, such as control structure or natural restriction. Emergency spillway sizing computations (based on 100-year storm event). 8. Flood routing through onsite conveyance and storage areas and through the first City structure. 9. Water surface profiles in the primary drainage system for each required design storm event(s). 10. Runoff peak rates and volumes discharged from the system for each required design storm event(s). 11. Tail water history and justification (time and elevation). b. Provide the results of any percolation tests, where appropriate, and soil borings that are representative of the actual site conditions, if requested by the City Engineer. c. Provide the acreage, and percentages of the total project, of the following: 1. Impervious surfaces, excluding wetlands. 2. Pervious surfaces (green areas not including wetlands). MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

24 1. MANUAL PURPOSE 3. Lakes, retention areas, other open water areas. 4. Wetlands. d. Provide an engineering analysis of floodplain storage and conveyance (if applicable), including the following: 1. Hydraulic calculations for all proposed traversing works. 2. Backwater water surface profiles showing upstream effects of traversing works. 3. Location and volume of encroachment within regulated floodplain(s). 4. Plan for compensating floodplain storage, if necessary, and calculations required for determining minimum building and road flood elevations. e. Provide a description of the engineering methodology, assumptions, and references for the parameters listed above, and a copy of all such computations, engineering plans, and specifications used to analyze the system. If a computer program is used for the analysis, provide the name of the program, description of the program, input and output data, and justification for model selection Operation and Maintenance and Legal Documentation a. Describe the overall maintenance and operation schedule for the proposed system. b. Identify the entity that will be responsible for operating and maintaining the system in perpetuity. c. Provide copies of all proposed conservation easements, storm water management system easements, property owner s association documents, and plats for the property containing the proposed system. d. Provide a copy of the boundary survey and/or legal description and acreage of the total land area of contiguous property owned or controlled by the applicant. 1.4 Example Problem An example problem was created to act as a supplement to the manual. The example problem has been divided into separate calculations and placed within the manual in the sections dealing with each particular component of the problem. The entire set of calculations have been summarized and are provided in Appendix A. The general overview of the problem is provided below and illustrated in Figure 1-2. A developer plans on converting a 10-acre parcel in the Auburn area into a subdivision. The parcel, as it stands, is in pasture (good condition), has Soil Type B, and a 1 percent average landscape slope from the northern end of the parcel to the outlet at the southern end a distance of approximately 1,000 ft. The developer s site plan for the subdivision is shown in Figure 1-2. Nine acres will be used for housing and roads, while 1 acre will contain a dry detention pond and common area. Housing density will be 4 houses per acre on the 9 acres containing housing. As illustrated, MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

25 1. MANUAL PURPOSE each acre, or subbasin, will be served by an inlet, with the main drainage line running along the road to the dry detention pond. The average subbasin grade toward each inlet will be 1 percent. The pond will discharge (free outlet) into an adjacent creek at the southern end of the property. The pond s bottom elevation should be set at elevation 90 ft 1. In accordance with the City s storm water management guidelines, the drainage system should be sized to handle the 25-year, 24-hour storm. The dry detention pond must be able to attenuate the 2-, 5-, 10-, and 25-year, 24-hour storm events to pre-development levels. The emergency spillway shall be designed to pass the 100-year, 24-hour storm event as defined in this manual. 1 Arbitrary reference elevation for illustration purposes. MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

26 A0.DM DESIGN INFORMATION Date: Project: Project Owner: Submitted by: Pond #1 Pond #2 Other Bottom Pond Elevation Maximum Pond Elevation during 25-yr Storm TopofPondEmbankment Pond Volume at Peak Pond Elevation (25- yr storm) Description of Outlet Outlet diameter/dimensions Orifice Invert elevation(s) Peak Outlflow 2-yr 5-yr 10-yr 25-yr Peak Pond Elevation 2-yr 5-yr 10-yr 25-yr Emergency Spillway Elevation Stage-Storage Relation * Routing* 100-yr storm provision Other * Attach additional information if necessary HSM-0075.CDR FIGURE 1-1A STANDARD SUBMITTAL FORMS DESIGN INFORMATION FORM Auburn Storm Water Management Manual

27 A0.DM PRE-DEVELOPMENT CONDITIONS Sub-Basin*: Total no. of Basins: Runoff Method: NRCS Rational Date: Project: Project Owner: Submitted by: 2-year storm 24-hour 5-year storm 24-hour 10-year storm 24-hour 25-year storm 24-hour Rainfall** Area(acres/miles 2 ) Curve Number/ Runoff Coefficient Average Slope over Critical path Total Length of Flow Path (ft) T c *** Intensity Peak flow (cfs) Runoff (inches) Street Drainage Checked Other * Use separate sheet for each sub-basin. ** If storms other than 24-hr event are used, attach pertinent information and justification. *** Attach all assumptions made in determining these values. HSM-0074.CDR FIGURE 1-1B STANDARD SUBMITTAL FORMS PRE-DEVELOPMENT CONDITIONS FORM Auburn Storm Water Management Manual

28 A0.DM POST-DEVELOPMENT CONDITIONS Sub-Basin*: Total no. of Basins: Runoff Method: NRCS Rational Date: Project: Project Owner: Submitted by: 2-year storm 24-hour 5-year storm 24-hour 10-year storm 24-hour 25-year storm 24-hour Rainfall (in)** Area(acres/miles 2 ) Curve Number/ Runoff Coefficient Average Slope over Critical path Total Length of Flow Path (ft) T c *** Intensity Peak flow (cfs) Runoff (inches) Street Drainage Checked Other * Use separate sheet for each sub-basin. ** If storms other than 24-hr event are used, attach pertinent information and justification. *** Attach all assumptions made in determining these values. HSM-0073.CDR FIGURE 1-1C STANDARD SUBMITTAL FORMS POST-DEVELOPMENT CONDITIONS FORM Auburn Storm Water Management Manual

29 A0.DM FT SCALE HSM-0061.CDR FIGURE 1-2 SW DRAINAGE SYSTEM AND SUBDIVISION LAYOUT Auburn Storm Water Management Manual

30 2. Runoff Estimation

31 2. Runoff Estimation Foremost in designing storm water conveyance and storage systems is to determine the volume and rate of runoff. The hydraulic design of a storm water conveyance system generally requires an estimate of the peak rate of runoff generated by the design event. In the following subsections, information will be provided to determine design storm information for the Auburn area, to predict excess precipitation, and to determine peak discharge rates. 2.1 Design Storm Information To determine the volume of runoff to be used for sizing facilities, design storms for the Auburn area must be determined. Design storms represent conservatively high rainfall intensities and volumes that can be expected during a storm event. Table 2-1 displays the rainfall intensities in inches per hour for a given return period and time of concentration for the Auburn area. Table 2-2 represents the depth of rainfall in inches that can be expected over a 24-hour period for a given return period. In addition to the total storm volume and peak intensities listed above, there is also a need to specify a time distribution of the intensities during the storm event. The City of Auburn uses the Natural Resource Conservation Service (NRCS) Type II dimensionless design storm. This information is to be used when computing runoff hydrographs utilizing computer models. It is widely used and typically is included in these types of models. 2.2 Excess Precipitation Calculation Excess precipitation is that portion of total precipitation that becomes storm water runoff during a storm event. The portion of a storm s total precipitation that does not become storm water runoff is called abstractions. Abstractions include interception infiltration, surface storage, surface detention, and bank storage. When a storm water system is designed using a single design storm event, initial abstractions need to be considered. Initial abstractions include interception and surface storage before runoff can begin. Once the initial abstractions are placated, then runoff occurs. The depth of runoff during a fixed time interval is the difference between the rainfall and infiltration capacity rates. Initial abstractions are insignificant compared to infiltration losses, and for practical hydrologic design, initial abstractions can be considered negligible. The infiltration rate is dependant on the physical properties of the soil, vegetative cover, antecedent soil moisture conditions, rainfall intensity, and slope of the infiltrating surface. This combination of factors that affects the infiltration rate for a particular watershed interacts to cause a generally complex spatial distribution of infiltration capacity. The NRCS Curve Number (CN) method may be used to determine the depth of runoff for a given area. MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

32 2. RUNOFF ESTIMATION NRCS Curve Number The NRCS CN method relates soil type, soil cover, land use type, and antecedent moisture conditions to a CN. Excess rainfall calculations are based on the CN and the event precipitation. The runoff volume is estimated by the following equation: where: (P 0.2S) 2 Q = (Equation 2-1) P + 0.8S Q = accumulated rainfall excess or runoff, inches P = accumulated rainfall, inches (refer to Table 2-2) S = maximum watershed rainfall retention factor The rainfall retention factor, S, includes the initial abstraction and infiltrated volume and is related to the watershed CN by the following relationship: 1,000 S = 10 (Equation 2-2) CN The watershed CN is a dimensionless coefficient that reflects watershed cover conditions, hydrologic soil group, land use, and antecedent moisture conditions. CN values for urban areas are provided in Table 2-3. Nearly all soils in Lee County are B-Type soils (Soil Survey of Lee County). B soils have fine to coarse texture moderately suited for infiltration and drainage. These values are to be used unless the applicant can prove to the City that the soils are different. Example 2.1: Pre-development Volume and Peak Discharge Determination Use the NRCS CN Method (Section 2.2.1) to determine the depth of runoff over the parcel. Table 2-3 indicates that pasture in good condition has a CN of 60. Using Equation 2-2: S = (1,000/60) 10 = 6.7 Using Equation 2-1 and the Design Storm Volumes from Table 2-2, the depth of runoff in inches can be calculated for each storm event. Multiply by the acreage to obtain the volume of runoff. For example: From Table 2-2 for a 2-year storm event, P = 4.2 inches. Q= [4.2 (0.2 * 6.7)] 2 /[4.2 + (0.8)6.7] = (2.86) 2 /9.56 = 0.86 inches Volume of Runoff = 0.86 inches * (ft/12 inches) * 10 acres * (43,560 ft 2 /acre) = 31,100 ft 3 Volume of Runoff = 31,100 ft 3 * ( gallons/ft 3 ) * (mg/1,000,000 gallons) = mg MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

33 2. RUNOFF ESTIMATION The calculations for each storm frequency are summarized in the following chart: Storm P (inches) Weighted CN S (inches) Q (inches) Volume of Runoff Acreage (acres) ft 3 mg 2-yr , yr , yr , yr , Notes: Runoff inches / 12 x Acreage x 43,560 ft²/ac = Volume ft³ ft³ x gallons/ft³ x 1/10 6 = Volume mg CN = Curve number mg = Million gallons ft 3 = Cubic feet 2.3 Storm Water Runoff Determination This subsection deals with estimating the peak storm water runoff rate that will flow from the watershed. An important step in determining the peak discharge is to also determine the distribution of all flows during the design storm, which will be discussed, as well Time of Concentration The time of concentration, t C, is the time required for water to travel from the most hydraulically distant point in the watershed to the watershed outlet. The TR-55 method for determining time of concentration will be used in this manual, with the following equation for time of concentration: where: t D = c 3600 (Equation 2-3) V t c = time of concentration, minutes D = distance runoff travels in watershed, ft V = average runoff velocity, feet per second (ft/sec) The average velocities can be computed using the following equations: V () s 0.5 up = (Equation 2-4) V () s 0.5 p = (Equation 2-5) MGM02-R3/017.DOC LATEST REVISION: APRIL 30,

34 2. RUNOFF ESTIMATION where: V up = average velocity for unpaved areas or channel flow, ft/sec V p = average velocity for paved areas, ft/sec s = slope of hydraulic grade line, feet per foot (ft/ft). Assume this equals the landscape slope. Figure 2-1 is a worksheet for determining the time of concentration that can be calculated for a watershed using the TR-55 method. The TR-55 method is commonly used to predict peak runoff for small watershed areas. This form also includes a section for computing the travel time for larger channels. If needed, the equations in Figure 2-1 can be applied to estimate channel velocity and the subsequent travel time. In general, larger channels will be modeled separately and would not be included in the subbasin drainage. To accurately determine the time of concentration, the flow path should be segmented as appropriate into sheet or overland flow (most recently determined not to exceed 100 ft, as documented in the new release of TR-55), shallow concentrated flow, and channelized flow. The time of concentration in each segment in the watershed should be calculated and then added together to get the total time of concentration. Pre- and post-development time of concentrations may differ significantly depending on the extent of development. In most cases, increased imperviousness and more direct flow routes decrease the time of concentration. where: t = t + t + t +...t (Equation 2-6) c c1 c2 c3 cn t cn = time of concentration for section n Peak Runoff Rate The preferred method for determining the peak runoff rate in urban settings is the Rational Method. However, use of the method will be limited to small contributing areas and t c values of 15 minutes and less. Otherwise, the TR-55 Method will be used. Refer to the flow chart in Figure 2-2 to determine which method to use. The well-known formula for the Rational Method is: Q T = CI A (Equation 2-7) T where: Q T = peak runoff rate, cubic feet per second (cfs), for the design storm return period, T C = rational method runoff coefficient expressed as a dimensionless ratio (Table 2-4) I T = average rainfall intensity, inches/hour (from Table 2-1), during a period of time equal to t c for the design storm return period, T. A = site drainage area, acres, tributary to the design point t c = watershed time of concentration, minutes, defined in Section MGM02-R3/017.DOC LATEST REVISION: APRIL 30,